[Identity] Use content to describe main olm account

Summary:
To avoid overloaded terms, use content instead of identity when
describing the main olm account.

Part of https://linear.app/comm/issue/ENG-3816

Test Plan: Gates pass

Reviewers: varun, ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7671